Such a known mechanism includes seven pedals with three positions, mounted on the base of the instrument and resiliently returned by springs, seven draw-rods mounted on the body of the instrument and fourteen systems of rods controlling each the rotation of the forks the two prongs of which, being normally clear of the string, coming to bear against one of the strings and twisting it, thereby creating a node of vibration. A large concert harp comprises 94 forks and each system of rods is formed of a return bell-crank lever displacing the rod, connection rocker bars acting on the cranks of the forks and return rocking arms for allowing the systems of rods to be housed in the saddle between two plates of the mechanism.
The major disadvantage of such a mechanism is the complexity of its transmission of movement with a number of parts, of which some are very small, being close to a thousand. This complexity results in a high cost and a very costly repair necessitating dismounting the mechanism into pieces since all the parts have to be disassembled, and this can take up to a hundred odd hours.
A further disadvantage is the operating noise resulting from the enormous number of parts which are set in motion and from the mechanical plays due to the high torque which has to be applied simultaneously on six or seven forks in order to twist strings stretched in a sufficient manner for creating a node of vibration.
A third disadvantage is mentioned in French patent No. 2 229 108 and results from the mode of operation of the forks which impart to the strings a stress leading to an elongation and a premature wear. In fact, this disadvantage is increased due to the fact that, because of the plays, the rotation movements of the seven forks of a same system of rods are not strictly performed over the same angle and the stress of the most twisted string is necessarily higher than that which would be necessary to provide the formation of a node in order that the fork having the greatest play provides nevertheless the necessary stress on its string. The above French patent proposes to remedy this disadvantage by pinching the string instead of twisting it, but this pinching, according to a well known principle in the stringed instruments field, is not usable in practice with the mechanism presently known for the harp without introducing, for each pinch, playcatch up devices which would make the mechanism even more complex and could not be housed in the saddle.
Another disavantage of the control of the forks through a mere rotational movement around an axis perpendicular to the string is that the clearance between the string and the parts of the fork or tongs at rest remains limited and there is a risk of contact between the vibrating string and said parts resulting in a so-called "zincking" noise.